Ticky-ticky profiling adds counters to every STG function. It's very low-level, but it really tells you what is going on:

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* Add the `-ticky` flag when compiling a Haskell module to enable "ticky-ticky" profiling of that module. This makes GHC emit performance-counting instructions in every STG function.

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* Add the `-ticky` flag when compiling a Haskell module to enable "ticky-ticky" profiling of that module. This makes GHC emit performance-counting instructions in every STG function.

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* Add `-ticky` to the command line when linking, so that you link against a version of the runtime system that allows you to display the results. In fact, in the link phase `-ticky` implies `-debug`, so you get the debug version of the runtime system too.

* Add `+RTS -rfoo.ticky` to the run-time command line, to put the ticky-ticky profile in the file `foo.ticky`.

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You need to use `-ddump-simpl -ddump-prep` when compiling the source files to see the functions that correspond to the performance counter report.

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You need to use `-ddump-prep` or `-ddump-stg` when compiling the source files to see the functions that correspond to the performance counter report.

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It's very low level stuff, but in exchange:

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* It's guaranteed that adding `-ticky` doesn't affect optimisation or transformation. It just adds the overhead of performance counters to the final code.

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* It's guaranteed that adding `-ticky` doesn't affect optimisation or transformation. It just adds the overhead of performance counters to the final code. (Any change these affect the CMM optimisations?)

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* You can mix modules compiled with `-ticky` and modules compiled without.

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To ''really'' see everything you need to compile all the libraries with `-ticky`. To do that in a standard build tree, here are some flag settings in `build.mk` that work:

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{{{

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# Build all libraries with -ticky

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GhcLibHcOpts += -ticky

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# Build the RTS in the ticky way

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GhcRTSWays += t

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# Currently ticky is incompatible with threading

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GhcThreaded = NO

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}}}

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== Ticky-ticky overview ==

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It is possible to compile Haskell programs so that they will count several kinds of interesting things, e.g., number of updates, number of data constructors entered, etc. We call this "ticky-ticky" profiling because that's the sound a CPU makes when it is running up all those counters (''slowly'').

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Ticky-ticky profiling is mainly intended for implementors; it is quite separate from the main "cost-centre" profiling system, intended for all users everywhere.

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You don't need to build GHC, the libraries, or the RTS a special way in order to use ticky-ticky profiling. You can decide on a module-by-module basis which parts of a program have the counters compiled in, using the compile-time `-ticky` option. Those modules that were not compiled with `-ticky` won't contribute to the ticky-ticky profiling results. That will normally include all the pre-compiled packages that your program links with.

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There are currently two coarse classes of ticky-ticky counters: name-specific counters and global counters.

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* name-specific counters

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Each "name-specific counter" is associated with a name that is defined in the result of !CorePrep. For each such name, there are three possible counters: entries, heap allocation by the named thing, and heap used to allocate that named thing.

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* global counters

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Each "global counter" describes some aspect of the entire program execution. For example, one global counter tracks total heap allocation; another tracks allocation for PAPs.

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== Enabling ticky-ticky and its extension flags ==

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Ticky-ticky counters are enabled in two ways.

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* A module compiled with `-ticky` will maintain name-specific counters for the names defined in that module's !CorePrep output.

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* A program linked with the `-debug` RTS will include the RTS's effect on the global ticky-ticky counters. At link-time, `-ticky` implies `-debug`.

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Some global counters are synthetized from multiple other counters, including both name-specific as well as other global counters. For example, the `ALLOC_HEAP_tot` counter accumulates the total of all heap allocations that were tracked by ticky-ticky; it is influenced both by the name-specific counters for allocation as well as by the global counters for heap allocation by the RTS.

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''By default'', the name-specific counters are only tracked for functions. In particular, both let-no-escape (LNE) names and thunks are not tracked. Allocation by each is included in the nearest lexically enclosing ticky counter. Entries in to each are not tracked at all.

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Two flags enable LNE and dynamically allocated thunks to be tracked by name-specific ticky counters: `-ticky-LNE` and `-ticky-dyn-thunk`. Note well that these flags, especially for dynamic thunks, incur much higher instrumentation overhead and much larger ticky reports.

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''By default'', the name-specific counters track only entries-into and allocation-by the named thing. A flag `-ticky-allocd` additionally tracks the heap used to allocate that thing. Again, this flag increases the instrumentation overhead.

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== Generating the ticky report ==

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Any GHC executable linked with `-rtsopts` will generate a ticky-ticky profiling report if provided the `-r` RTS option. This report includes all global counters as well as the name-specific counters for those names with at least one interesting counter value. If a named thing was never allocated and (hence) never entered, its counters will not be in the ticky report.

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Below is an excerpt from a ticky report. The executable was compiled with all of the extensions above.

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The first three columns show the three name-specific counters: entries, allocation-by, and allocation-of. The fourth column gives a short summary of the named things's non-void arguments: how many there are and a terse description of each, according to the following table.

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|| Classification || Description ||

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|| `+` || dictionary ||

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|| `>` || function ||

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|| `{C,I,F,D,W}` || char, int, float, double, word ||

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|| `{c,i,f,d,w}` || unboxed ditto ||

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|| `T` || tuple ||

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|| `P` || other primitive type ||

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|| `p` || unboxed ditto ||

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|| `L` || list ||

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|| `E` || enumeration type ||

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|| `S` || other single-constructor type ||

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|| `M` || other multi-constructor data-con type ||

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|| `.` || other type ||

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|| `-` || reserved for others to mark as "uninteresting" ||

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The final column is the !CorePrep/STG name to which the counters in this row refer. Each entry in this column uses an encoding that differentiations between exported names (`main:Main.puzzle`) and non-exported names (`go1{v r2Hj} (main:Main)`). Some non-exported names indicate that they are let-no-escape (`(LNE)`) or a dynamically allocated thunk (`(thk)`). All let-bound names also specify the unique of the parent (`in s2T4`). The "parent", here, is the innermost enclosing definition that has a ticky counter; the parent is affected by `-ticky-LNE` and `-ticky-dyn-thunk`).

The formatting of the information above the row of asterisks is subject to change, but hopefully provides a useful human-readable summary. Below the asterisks ''all counters'' maintained by the ticky-ticky system are dumped, in a format intended to be machine-readable: zero or more spaces, an integer, a space, the counter name, and a newline.

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In fact, not ''all'' counters are necessarily dumped; compile- or run-time flags can render certain counters invalid. In this case, either the counter will simply not appear, or it will appear with a modified counter name, possibly along with an explanation for the omission. Software analysing this output should always check that it has the counters it expects. Also, beware: some of the counters can have very large values.

If some of the counters are zero when they shouldn't be, that means they're not implemented yet. If you want them to be, complain on a mailing list.

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== Implementation notes ==

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When compiling with {{{-ticky}}}, the back-end emits the name-specific counters in the data section and generates instructions that tick them directly.

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The global counters are ''always'' statically allocated by the RTS [[GhcFile(includes/stg/Ticky.h)]] regardless of the compilation way.

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Some `TICK_...` C-- macros are sprinkled throughout the RTS. In the debug way (cf `ways.mk`) `TICKY_TICKY` is `defined`, so these macros expand to code that ticks the counters. The relevant compiler code is mostly in [[GhcFile(compiler/codeGen/StgCmmTicky.hs)]]. NB that some of these macros are expanded by [[GhcFile(compiler/cmm/CmmParse.y)]]!

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The C-- macros are defined in [[GhcFile(includes/Cmm.h)]]. Most of them (probably all of them, at the moment) just increment counters (variables in C) that are declared in [[GhcFile(includes/stg/Ticky.h)]]. The latter file is likely to get out of sync with the former, so it really should be automatically generated.

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The code in the RTS that prints out the ticky report is in [[GhcFile(rts/Ticky.c)]].